Sodalites fall into the category of cage type aluminosilicates in which aluminum and silicon ions are tetrahedrally surrounded by oxygen ions to form a three-dimensional aluminosilicate cage structure with cubic octahedral cavities. Part of these cavities, or all of them, are filled with halogen ions tetrahedrally surrounded by alkali metal ions. The degree of filling the sodalite cage cavities is indicative of the amount of nonstoichiometry of the sodalite structure which is the main governing factor controlling the photochromic and cathodochromic properties of sodalites. In this case, the 100%-filling of the sodalite cage cavities with alkali-halide tetrahedra is equivalent to stoichiometric form, and correspondingly, to zero amount of nonsoichiometry. The crystal lattice of the sodalite cage structure has cubic symmetry, and hence the sodalite crystals are optically isotropic.
The crystalline structure of sodalite in the form of mineral chlorosodalite was first thoroughly investigated and described by L. Pauling in 1930 (L. Pauling, "The structure of Sodalite and Helvite", Zeitschrift fur Kristallografie, v.74, 1930, p.213). In the natural chlorosodalite studied by L. Pauling, which has an idealized chemical formula Na.sub.6 Al.sub.6 Si.sub.6 O.sub.24.2(NaCl), between 85 and 90% of cavities of the aluminosilicate cage are filled with ClNa.sub.4 -tetrahedra. The lattice constant of the aluminosilicate cage of this sodalite is 8.87 .ANG., and, correspondingly, the volume of each cubic octahedral cavity is about 150 .ANG..sup.3.
On discovering photochromic, and subsequently, cathodochromic properties in natural chlorosodalites, some attempts were made to use these materials for manufacturing the screens of cathodochromic CR storage tubes known as the sciatrons. The industrial application of natural sodalites in optoelectronics, however, was limited by the shortage of raw materials, high cost of extraction and processing, high proportion of impurities, low nonstoichiometry value of the composition, and other causes. Consequently, the development of synthetic sodalite materials was initiated resulting in a wide range of sodalites of different chemical compositions.
The known synthetic sodalite materials widely vary in their chemical composition. So, for example, sodalite materials have been synthesized in which part of the Al.sup.3+ and Si.sup.4+ ions of the aluminosilicate cage is repplaced by Ga.sup.3+ and Ge.sup.4+ ions, respectively. Known are synthetic sodalite materials which, unlike the previously described chlorosodalite, contain ions of halogens other than chlorine (Cl.sup.-), namely: fluorine F.sup.-, bromine Br.sup.-, and iodine I.sup.-, separately or in combination, and part of the sodium ions (Na.sup.+) is replaced by ions of other alkali or alkaline-earth metals. Some synthetic sodalite materials may contain alloying additions such as SO.sub.4.sup.2, S.sup.2-, Se.sup.2-, Te.sup.2-, WO.sub.4.sup.2-, etc. The desired photochromic and cathodochromic properties of sodalite materials are provided by varying the chemical composition. Here the crystalline structure of sodalites remains unaltered, but the lattice constant of the aluminosilicate cage is liable to change, generally increasing compared to that indicated above for mineral chlorosodalite, which leads to a shift in the colour center absorption band, resulting in turn, in changed photochromic and cathodochromic characteristics.
These changes in chemical composition are generally realized in sodalite powders, which is the most commonly used form of photochromic or cathodochromic sodalite materials. Such synthetic sodalite powders are mostly prepared by the solid-phase sintering method, the low-temperature hydrothermal method, or the zeolite conversion method (B. W. Faughnan, J. Gorog, P. M. Heyman, J. Shidlovsky, "Cathodochromic Materials and Applications", Proceeding of the IEEE, v.61, No.7, 1973, pp.927-941). In implementing these methods, the starting materials are elements or element compounds providing the desired chemical composition of sodalites. In order to increase the photochromic and cathodochromic sensitivity of synthetic sodalite powders, they are sensitized by heat treatment at a temperature of from 600.degree. to 1000.degree. C. for a period ranging from tens of minutes to a few hours (U.S. Pat. No. 3,799,881, published in 1976). Such treatment removes a certain part of alkali halides from the aluminosilicate cage structure cavities, making it possible to reach the desired amount of nonstoichiometry of the sodalite composition, lying within the range 5-70%.
As the synthetic sodalite powders are employed for manufacture of sensing elements of variable light transmission devices, a sensitive layer is formed by deposition, evaporation, or sintering of the sodalite powder on a rigid substrate. The synthetic sodalite powders enable us to produce sensing elements exhibiting good photochromic or cathodochromic properties. The powder form of the materials considered, however, gives rise to heavy diffuse light scattering in sensitive layers of such materials, and consequently, to a low optical transparency. It presents difficulties in the readout of the stored information by projecting it onto the screen, since the use of a simple projection system with the light flux passing through the sensing element results in a low-resolution and low-contrast image. In order to minimize the effect of diffuse light scattering, a variable light transmission device was proposed permitting the information readout from the same surface of the sensitive layer that was used for the information storage, while the projection of the image is carried out in the reflected light flux (L. T. Todd, C. J. Starkey, "High brightness, high resolution projection CCRT", 1977, International Electron Devices Meeting, IEEE, New York, 1977, pp.80A-D). Such a device is largely complicated, and also it fails to eliminate the effect of the diffuse light scattering, due to the sensitive layer surface, on the quality of the image projected.
The presence of the diffuse light scattering in the sensitive layer of sodalite powder prevents the use of the coherent light for reading and writing the information, thus substantially limiting the areas of applications of the sensing elements based on sodalite powder. Further, the inherently large surface area of powder materials brings about an increased adsorptive capacity with respect to water molecules and hydroxyl groups. The large number of vacancies in the aluminosilicate cage structure of sodalites also contributes to this effect. Therefore, considerable amounts of water, in the form of molecules or hydroxyl groups, are accumulated with time in the sensitive layers of sodalite powders, even in those subjected to dehydration, resulting in a reduced sensitivity of the sodalite as a registration medium. Furthermore, the manufacture of sensing elements using sodalite powders is rather complicated, since measures to avoid penetration of impurities should be taken, as the sensitive layer is formed. The need in a rigid substrate when using the sodalite powders gives rise to certain problems in the choice of materials capable of resisting high-energy radiation, while providing a strong mechanical coupling with the sensitive layer formed on the substrate.
In order that light transmission of the sensitive layer of synthetic sodalite powders be improved, it was proposed to add 50-70 weight percent of aluminum phosphate (USSR Inventor's Certificate No. 674116, published in 1979). In this manner, an immersion medium was expected to be produced in the sensitive layer, having sodalite crystals distributed therein and exhibiting a refractive index close to that of the sodalite. However, the decrease in diffuse light scattering proved to be very small, while radiation resistance of such a sensitive layer compared to sensitive layers consisting of sodalite alone was severely impaired due to mechanical electron-beam induced microdefects. In addition, there was a noticeable increase in electron beam energy absorption together with a number of other unwanted effects.
In addition to synthetic sodalite powders, there are photochromic and cathodochromic sodalite materials in the form of single crystals grown by the high-temperature hydrothermal method followed by radiation sensitization (cf. USSR Inventor's Certificate No. 400137 published in 1974). The chemical composition of such sodalites may be expressed by the formula of hydrosodalite Na.sub.6 Al.sub.6 Si.sub.6 O.sub.24.2 (NaOH).3H.sub.2 O). In this case, as much as 20-30% of hydroxyl groups can be replaced by halogen ions or other alloying additions. Single crystals of sodalite possess a high optical transparency, but a sufficiently high amount of nonstoichiometry and consequently, satisfactory photochromic and cathodochromic properties have not been achieved so far. Besides, the known method of sodalite single crystal growth fails to provide sodalites of an optimum chemical composition comparable to that encountered in synthetic sodalite powders exhibiting the maximum photochromic and cathodochromic sensitivity. At the same time, the process of synthesizing monocrystaline sodalite is very cumbersome, expensive, and time-consuming, and the single crystals are produced in sizes not exceeding a few centimeters and cannot be employed as sensing elements for variable light transmission devices, considering their nonuniformity.
U.S. Pat. No. 3,923,529 published in 1975 deals with sodalite-like photochromic materials in the form of glasses or glassceramic. In accordance with this patent, the glasses are prepared by processing the mixture providing a chemical composition of the glasses close to that of the sodalites. Glass materials, however, basically differ from sodalites in that they have an amorphous structure. Glassceramic, according to the patent, was fabricated from glass by means of heat treatment to provide inner crystallization in glass to form sodalite crystals and similar crystalline phases. Such glassceramic in the monolithic form including sodalite crystal particles can be thought of as the nearest counterpart of the present invention.
Like sodalite single crystals, the sodalite-like glasses are optically transparent and not water-adsorbing. In this case, sufficiently large glass sizes may be available as compared to those produced by synthesis of single crystals of sodalite. As apparent from the data, the photochromic sensitivity of sodalite-like glasses produced, however, is as low as that of the sodalite single crystals, and the evidence of their cathodochromic sensitivity is not available as yet. In fact, it is basically impossible to achieve satisfactory photochromic or cathodochromic properties in sodalite-like glasses. It is due to inherently amorphous structure of the glasses, being deprived of a crystalline cage with isolated cavities that are indispensable for generation of F colour centres. Glassceramic comprising sodalite crystals distributed in the amorphous phase, as evidenced by the data, shows a higher photochromatic sensitivity. Yet glassceramic is not optically transparent. Furthermore, the method of its preparation comprising the known glass manufacturing process characterized by high temperature parameters makes it impossible to obtain a sufficiently high content of high-volatile halogens in glass and hence, in glassceramic fabricated therefrom. It prevents satisfactory photochromic or cathodochromic properties from being attained in sodalite glassceramic.
As may be inferred from the above description of the prior art, of all the known sodalite materials, only synthetic sodalite powders can be employed for preparation of the sensing elements of variable light transmission devices, these powders being used to form a sensitive layer on a rigid substrate. Such sensitive layers, however, fail to provide satisfactory optical performance, as was mentioned above.